I was registering for a stack overflow account and encountered a problem.
I was using a sound captcha. No account was created when I submitted the form, but I was able to (obviously) create an account using the text captcha.
This probably had nothing to do with the sound captcha, actually. It looks like openid.se decided you were already logged in for some reason, not entirely sure what happened there.
I've deployed some potential fixes, and more logging (and better user facing error messages, "Not Found" is garbage I apologize for).
I had the exact same problem. I simply got the error you get. It was a cookies problem. For the duration of the sign in, you have to allow everything.